Optical parametric oscillators (OPOs) use parametric frequency conversion in a nonlinear material to convert input electromagnetic radiation of a short wavelength to tunable output electromagnetic radiation at longer wavelengths. The input electromagnetic wave is called the pump-wave and the two outputs thereby generated are typically called the signal-wave and the idler wave, by usual convention the wavelength of the signal-wave being the shorter of the two wavelengths. These two outputs when considered together are by convention referred to as the down-converted waves or the down-converted radiation. In this way, OPOs take a high-energy, i.e. short wavelength, photon and divide its energy between two newly generated lower-energy, i.e. longer wavelength, photons.
An optical parametric oscillator in general consists of an optically nonlinear material which is located appropriately within an optical cavity that (i) resonates either the signal-wave or the idler-wave (singly-resonant oscillator), or (ii) resonates both waves simultaneously (doubly-resonant oscillator). In this latter case, two cavities may be employed, one to resonate the signal-wave, the other to resonate the idler-wave. For singly resonant oscillators, usually the cavity has a low loss at the appropriate signal or idler wavelength, and the nonlinear material is phase-matched to efficiently generate light at the correct wavelength. An optical parametric oscillator has to be pumped by a wave or radiation from a pump-laser. The pump-laser in general has a gain-medium for generating the pump-wave. The gain-medium is usually incorporated within the optical cavity of the pump-laser that resonates the pump-wave. The gain-medium of the pump-laser generally has to be excited by some external source of power/energy, for example another laser, such as a diode-laser.
OPOs are flexible sources of coherent radiation that can be tuned over substantial bandwidths in the ultraviolet, visible, infrared and terahertz spectral regions. It will be appreciated that the generic term “optical” as used throughout is taken as embracing all of these spectral regions. Examples of OPOs are described in the articles “Continuous-wave, singly-resonant intra-cavity optical parametric oscillator based on periodically-poled LiNbO3”, by Turnbull et al, Electronics Letters 33(21), pages 1817-1818 (1997); “Widely Tunable all-solid-state optical parametric oscillator for the visible and near infrared” by Cui et al, Optics Letters 18(2), pages 122-124 (1993), and “Tunable ultraviolet optical parametric oscillator for differential absorption lidar measurements of tropospheric ozone” by Fix et al, Applied Physics B 75(2-3), pages 153-163 (2002).
OPOs have been operated on many timescales from the femtosecond pulse to the true continuous-wave. The advent of new nonlinear materials, in particular quasi-phase-matched nonlinear materials, has contributed significantly to these devices becoming practical sources. In quasi-phase-matched nonlinear materials the crystal domain structure is periodically reversed; by way of examples this being brought about either through periodic-poling, where the domains can be periodically reversed by applying a high voltage across the crystal through a patterned electrode, or through configurationally-oriented crystal growth. By varying the periodicity of the domain pattern in the crystal, the wavelengths of the signal-wave and idler-wave, which are phase-matched to a given pump wavelength, can be changed.
Despite the advent of quasi-phase-matched nonlinear materials, problems with the practicality of OPOs still exist, particularly in the case of continuous-wave devices. A particular problem, which restricts development of compact/miniature devices, is that substantial pump-wave intensities and hence pump-wave powers are required for the parametric oscillator to reach oscillation threshold. One solution to the high threshold problem is to put the optical parametric oscillator, and in particular the nonlinear material of the OPO, within the cavity of the pump-laser. Under the condition of the gain-medium of the pump-laser being able to deliver a given pump-wave power, the pump-wave intensity within the cavity of the pump-laser can be significantly higher, typically by factors greater the ten, than the pump-wave intensity that can be coupled out of the cavity, and therefore oscillation threshold can be reached with a much lower pump-wave power if the intra-cavity arrangement is employed. This type of device is known as an intra-cavity optical parametric oscillator. Such a device has been described by a number of authors, see in particular “Continuous-wave, singly-resonant, inter-cavity parametric oscillator” by Colville et al, Optics Letters 22(2), pages 75-77 (1997); “Optical parametric devices and processes” by Ebrahimzadeh, JOSA B 16(9), page 1477 (1999); “Parametric generation of tunable light from continuous-wave to femtosecond pulses” by Dunn et al, Science 286(5444), pages 1513-1517 (1999), and “Internal optical parametric oscillators”, by Oshman et al, IEEE, J. Quantum Electronics QE-4, pages 491-502 (1968).
FIG. 1 shows an example of a known continuous-wave intra-cavity optical parametric oscillator (see for example “Low-pump-threshold continuous-wave singly resonant optical parametric oscillator”, D. J. M. Stothard, M. Ebrahimzadeh, and M. H. Dunn, Optics Letters 23, 1895-97 (1998)). This has a gain-medium 14 into which radiation from the semiconductor laser-diode 10 is directed by way of a lens arrangement 12 for the purpose of exciting the gain-medium. The lens 12 is provided for optimally matching the spatial profile of the radiation from the laser-diode 10 to the mode size, preferably the fundamental mode, of the radiation in the gain-medium 14. As a specific example, the laser gain-medium 14 is neodymium:vanadate, and the diode-laser 10 is adapted to deliver one watt of optical power at 809 nanometers, a wavelength at which there is a strong absorption feature associated with neodymium:vanadate.
On a back surface of the gain-medium 14, and integral with it, is a reflective material that defines a first mirror 16. Opposite the gain-medium 14 is a second reflective surface 18. Between the laser gain-medium 14 and the second reflective surface 18, and along an optical axis thereof, are in sequence a lens 20, a beam-splitter 22 and a nonlinear material 24, in this case a periodically poled lithium niobate (PPLN) crystal that is about 50 mm long and has a grating period of 29.3 microns. The purpose of the lens 20 is to enable the appropriate mode sizes to be obtained in the laser gain-medium 14 and the nonlinear material 24, when used in association with the first and second mirrors 16 and 18. Off the main optical axis is provided a third mirror 26, which is positioned so that light reflected from the beam-splitter 22 is directed onto it.
Each of the first and second mirrors 16 and 18, which define the cavity of the pump-laser, is highly reflective at the wavelength of the light, the pump-wave, emitted from the laser gain-medium 14. The beam splitter 22 is highly transmissive at the wavelength of the pump-wave so that it allows light emitted from the gain-medium 14 to pass through it and into the nonlinear material 24, whilst at the same time is highly reflective to down-converted waves emitted from the nonlinear material 24 so as to reflect such radiation either onto the third mirror 26 or back into the nonlinear material 24. A number of combinations of reflectivities of the second and third mirrors at the signal and idler wavelengths exist depending on which or both are the resonant waves. In this case, the second mirror 18 is wholly reflective at the signal wavelength, as well as at the pump wavelength as aforementioned, while being wholly transmissive at the idler wavelength so that an output can be gained. The third mirror 26 is wholly reflective to the down converted light, both at the signal-wave and idler-wave wavelengths, emitted from the nonlinear material. This configuration relates to a singly resonant OPO with regard to the down-converted waves, being resonant for the signal-wave only, and in which the idler-wave double passes the nonlinear medium. A number of variants on the above described particular design are possible while still retaining the concept of an intra-cavity OPO in which the OPO in whole or in part is located within the cavity of the pump-laser.
The arrangement of FIG. 1 has two coupled cavities, namely the cavity of the pump-laser defined by the optical path between the first and second mirrors 16 and 18, in which the nonlinear material 24 is located along with the gain-medium 14 of the pump-laser itself, and a second cavity, defined by the optical path between the second and third mirrors 18 and 26, in which the nonlinear material 24 is also located and which is associated with the resonant wave of the down-converted coherent radiation generated by this nonlinear material 24, and which is referred to previously as the cavity of the OPO. The two cavities are coupled through the nonlinear material 24.
When the arrangement of FIG. 1 is used, continuous stimulation of the nonlinear material 24 by radiation generated by the gain-medium 14 causes an optical parametric down conversion process to start and so generates a pair of continuous-wave signal- and idler-waves. In practice it has been found that the intensity stability of both the intra-cavity pump-field/wave and the intra-cavity signal/idler-field/wave are compromised when the parametric down conversion process is present extending over a range of timescales including: (i) oscillatory behaviour, with oscillation periods typically in the range 10−7 to 10−5 s; (ii) rapid growth coupled with slow decay of the envelope of oscillations, where damping times may exceed 10−3 seconds; and (iii) where the oscillations can become essentially continuous through being repeatedly triggered on timescales of the order of the damping time. This can be seen in FIG. 2, which shows the temporal profile of the intra-cavity pump-field recorded by a photodiode that has a response time that is significantly less than the oscillation period.
Also shown in FIG. 2 is the intra-cavity pump-field when the down conversion process provided by the optical parametric oscillator is inhibited, for example, by placing a shutter between the beam-splitter 22 and the third mirror 26. In this case the pump-field exhibits stable operation. Hence, the inclusion of the intra-cavity parametric oscillator within the laser cavity significantly modifies the dynamics of the intra-cavity pump-field in the form of a relaxation oscillation type of behaviour, most notably the magnitude, period and decay time of these oscillations. The signal-field and the idler-field of the OPO exhibit similar effects.
As is well known, the occurrence of relaxation oscillations can prove detrimental to the operation of an intra-cavity continuous-wave optical parametric oscillator as a stable source in terms of both amplitude and frequency stability of the coherent radiation generated. This is discussed in the articles “Continuous-wave intracavity optical parametric oscillators: an analysis of power characteristics”, by Turnbull et al, Applied Physics B 66, pages 701-710 (1998) and “Transient dynamics of CW intracavity singly resonant optical parametric oscillators”, by Turnbull et al, IEEE, Journal of Quantum Electronics 35(11), pages 1666-1672 (1999).
Relaxation oscillations are widely known in laser devices. For example, such relaxations are widely known in the case of neodymium lasers and semiconductor lasers, see “Output fluctuations of CW-pumped Nd: YAG lasers”, by Koechner, IEEE Journal of Quantum Electronics QE-8(7), pages 656-661 (1972), and “Relaxation oscillations in quasi-single-mode semiconductor lasers”, by Zaibel et al, IEEE Journal of Quantum Electronics 3(9), pages 2081-2086 (1994).
In the case of intra-cavity optical parametric oscillators, where two coupled cavities are involved, the dynamics of the oscillatory behaviour is different in kind from that encountered in basic laser devices. By way of an illustration, the period of the oscillations in the intra-cavity OPO is determined pre-dominantly by the decay time of the pump-wave radiation within the passive cavity of the pump-laser or the decay time of the resonant signal/idler-wave radiation within the passive cavity of the OPO. It has been shown, both experimentally and theoretically, that the effects of relaxation oscillations are particularly severe for intra-cavity OPOs; see previous references to Turnbull et al. These relaxation oscillations can be triggered by many different mechanisms, for example thermal effects in the nonlinear medium and interferometric feedback. Thus, they present significant problems with regard to the operation of continuous-wave intra-cavity optical parametric oscillators.
As well as problems with relaxation oscillations, an OPO with a pump-laser based on a neodymium gain-medium, or indeed other solid state gain medium, faces a number of other drawbacks. For example, neodymium based pump-lasers exhibit the phenomenon of spatial hole burning which leads to a significant problem in obtaining single-frequency (single axial mode) oscillation when a standing-wave cavity is employed. Also, they lack flexibility with regard to spectral coverage, as the laser transitions are confined to the range 0.9 to 1.5 μm. Furthermore, they exhibit thermal lensing effects at higher powers, leading to stability and reliability problems, and have a restricted continuous tuning range for the down-converted waves, when done using continuous tuning of the pump-wave, due to their narrow gain bandwidth.